health: fix nil DERPMap dereference panic

Looks like a DERPmap might not be available when we try to get the
name associated with a region ID, and that was causing an intermittent
panic in CI.

Fixes #12534

Change-Id: I4ace53681bf004df46c728cff830b27339254243
Signed-off-by: Andrea Gottardo <andrea@gottardo.me>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f